The present invention relates to a method of producing CRT funnel glass which connects panel glass for displaying a video image and neck glass equipped with an electron gun for projecting the video image.
An envelope of a CRT comprises a panel portion for projecting video images, a tubular neck portion with an electron gun arranged therein, and a flare-shaped funnel portion connecting the panel portion and the neck portion with each other. The panel portion, the neck portion and the funnel portion are in the form of glass members, respectively. Electron beams emitted from the electron gun excite phosphors arranged on an inner surface of the panel portion to emit light so that the video images are projected on the panel portion. At this time, X-rays bremsstrahlung are produced inside the CRT. X-rays bremsstrahlung adversely affect human bodies when leaked to the exterior of the CRT through the envelope. Therefore, the envelope of this type is required to have a high X-ray absorbability.
CRTs are classified broadly into a black-and-white CRT bulb and a color CRT bulb. Generally, a panel portion of the black-and-white CRT bulb (hereinafter referred to as “black-and-white panel”) is required that its glass is not colored due to electron beams and X-rays. The black-and-white panel is made of glass containing about 5 mass % of PbO. A panel portion of the color CRT bulb (hereinafter referred to as “color panel”) is used at higher voltages as compared with the black-and-white panel, and thus is required more severely that its glass is not colored due to electron beams and X-rays. Therefore, the color panel is made of glass containing no PbO but containing SrO and BaO.
Since the neck portion is small in thickness, it is required to have a higher X-ray absorption coefficient. Therefore, the neck portion is made of glass containing no less than 20 mass % of PbO.
For equalizing the potential within a CRT bulb upon emitting electron beams from an electron gun, the funnel portion is coated with a carbon conductive film (carbon DAG) on an inner and an outer surface thereof. Thus, in addition to having a high X-ray absorbability, the funnel portion is also required to have a water durability to an extent for preventing contamination of its glass surface due to alkali effusion. Therefore, the funnel portion is made of glass containing no less than 10 mass % of PbO and about 5 mass % of MgO+CaO.
The panel portion and the funnel portion are joined together by sealing in case of the black-and-white CRT bulb, and by a PbO frit made of glass containing PbO in case of the color CRT bulb. The funnel portion and the neck portion are joined together by sealing.
On the other hand, in recent years, recycling of CRT bulb glass members has been advanced. Specifically, glass members of CRT bulbs are first classified into black-and-white bulb glass members and color bulb glass members, which are then further classified into groups of panel glass members, funnel glass members and neck glass members. Then, the glass members are immersed per group into a chemical liquid to remove phosphors, carbon DAGs, frits and so on. Subsequently, the glass members are comminuted per group and the comminuted glass members are used as a glass material for producing glass members of the same group, so that recycling is carried out.
The reason why the glass members should be strictly classified as described above is that, for example, if the glass members containing PbO, such as the black-and-white panels, are mixed into a glass material of the color panels, the color panels made of this glass material are subjected to coloration due to electron beams and X-rays.
Further, if the frits are not removed completely from the color panels, the same problem will be raised.
However, the complete classification of the glass members per group as described above requires much time and labor, and thus leads to the increase in production cost. Further, there is also a problem that a demand for the black-and-white CRT bulb glass is small and thus the production amount thereof is limited, so that complete recycling can not be achieved with respect to the black-and-white CRT bulb glass.
In view of the above, the color panels with the adhering frits, the black-and-white panels and the neck glass members with the attached funnel glass members are now being recycled as a material of the funnel glass which is PbO glass as described above.
On the other hand, the black-and-white panels, the color panels and the neck glass members contain large amounts of SrO and BaO. Thus, if such recycling is performed, SrO and BaO are mixed into the funnel glass. Further, since the amounts of the black-and-white panels, the color panels and the neck glass members to be recycled are not fixed, the rate of the recycled glass contained in a material of the funnel glass fluctuates and, following this, the amounts of contained SrO and BaO also-fluctuate.
If SrO and BaO are mixed into the funnel glass, a thermal expansion coefficient and an annealing point of the glass are increased, while a molding temperature thereof is lowered. The increase of the thermal expansion coefficient or annealing point of the glass causes generation of a stress beyond an allowable range when sealing is performed between a panel glass member and a funnel glass member or between a funnel glass member and a neck glass member, thereby to lower the strength of a CRT bulb. Further, the fluctuation of the contained amounts of SrO and BaO causes the molding temperature to be unconstant so that the temperature condition upon forming the glass fluctuates to induce lowering of the productivity.
Even if the content of Al2O3, PbO or alkali metal oxide (Na2O, K2O) in the glass is adjusted in an attempt to correct the glass characteristics which have been changed due to entering of SrO and BaC, it is hardly possible to lower the thermal expansion coefficient or annealing point of the glass, or increase the molding temperature thereof. In view of this, in general, the content of MgO or CaO in the glass is reduced to prevent these characteristics from changing.
However, if the content of MaO or CaO is reduced, the water durability of the glass is deteriorated, so that alkali effusion tends to occur while keeping the funnel glass in the form of funnel glass members, resulting in contamination of the surfaces thereof. If a carbon DAG is applied to such a funnel glass member, DAG repelling occurs at contaminated portions so that the carbon DAG can not be applied uniformly. As a result, the potential within a CRT bulb can not be equalized, meaning that such a funnel glass member has a critical defect for forming the funnel portion of the CRT bulb.
If the surface of the funnel glass member is washed using strong acid before applying the carbon DAG, the DAG repelling can be suppressed. However, it is not desirable in view of environmental sanitation and in view of increased cost due to the increase in production process.
Further, there has been developed a technique wherein an organic or inorganic water soluble protective film is applied to the glass surface immediately after forming the funnel glass member, then the glass surface is washed with water immediately before applying the carbon DAG, thereby to remove the film and contaminants adhered thereon. However, if such a funnel glass member is kept over a long term, it is possible that alkali effusion occurs.